Thermal stress to components containing or comprising materials of different thermal expansion coefficients is a well-known problem. Within the art of making wind turbines, this problem is particularly pronounced due to the very varying operational and climatic conditions, that many wind turbines are exposed to. Especially electric components, such as the generators of wind turbines, are vulnerable to thermal stress.
Basically, thermal stress originates from two factors, namely high temperatures and, more important, varying temperatures.
Overheating of the windings of a generator does not only reduce the lifetime of the windings due to chemical decomposition of the insulating materials but can also lead to more immediate damage to or even destruction of the windings.
The lifetime of the insulating material depends strongly on the temperature of the material, and, roughly speaking, the lifetime is halved by a temperature raise of approximately 10° C. This is in good accordance with Arrhenius' exponential “law”, which is a well-proven theory suggesting that the higher the temperature, the faster a given chemical reaction will proceed. For electrical components, a rule of thumb says that for every 10° C. the temperature is raised, the risk of failures doubles.
Even more important, varying temperatures result in consecutive extensions and contractions of the mechanical parts of the electrical components, which can eventually lead to fatigue of the materials constituting the parts and, thereby, damage to or destruction of the electrical components.
Furthermore, the lifetime of generator windings is reduced because different thermal expansion coefficients of the conducting material, the insulating material and the material surrounding the windings result in decomposition due to mechanical wear of the different materials as they slide against each other, because they expand differently when the temperature changes. Similar effects apply for cables that are exposed to varying temperatures.
The Coffin-Manson model, which is, for instance, described in International Patent Application WO 2007/051464, further discusses some of the relations between temperature variations and lifetime of a material.
Also, the fact, that lubricants and interacting mechanical components are typically made to work optimally at a specific temperature, influences the lifetime of components which are exposed to significant temperature variations. If lubricants are used at temperatures outside the temperature ranges, within which they are made to work optimally, the friction between different lubricated materials as they slide against each other, because they expand differently when the temperature changes and, thereby, the mechanical wear of the materials may be increased.
For the above-mentioned reasons, it is seen that not only should the temperature of the different components, especially the electrical components such as the windings of the generator, be kept below a specified maximum temperature but, optimally, it should be maintained at a fixed predefined, optimal operational temperature.
If this should be accomplished by a heating and cooling system, however, the capacity of this system should be extremely high, and the system would be very expensive in manufacturing and operation as well as in maintenance costs. Furthermore, such a heating and cooling system would be both large and heavy, which is particularly disadvantageous in the field of wind turbines.
Therefore, some kind of control of the operation of the wind turbine is required in order to reduce the thermal stress of the components of the wind turbine.
A normal control strategy for this purpose is to monitor a series of parameters, such as ambient temperature, temperatures of the stator, bearings and cooling fluid of the generator, reactive power production, rotor currents, undervoltage and asymmetric phases on the utility grid, each of which parameters affects the temperatures of the generator.
For instance, it is a well-known and normal procedure to monitor the temperature of the stator in order to be able to intervene by reducing the magnitude of the currents running in the stator before overheating occurs. This monitoring is usually performed using temperature sensors, such as PT100 sensors or other temperature dependent resistors physically positioned within the stator from where the sensor output reaches a control system through simple wiring.
In a similar way, temperatures in the bearings and the cooling fluid of the generator are measured using temperature sensors, typically PT100 and/or the like.
The use of temperature sensors of this type, however, is not very useful for monitoring the rotor temperature because the rotation of the rotor complicates the transmission of the sensor output to a control system. Normally, measurement signals are transmitted from the rotor to the stationary part of the generator through a system of slip rings, but the electrical resistances of such a slip ring system are not constant.
In fact, the variations of the slip ring resistances may, in severe cases, exceed the variations of the resistances of the temperature dependent resistors which makes the use of slip rings unsuitable for rotor temperature monitoring.
Generally, direct rotor temperature monitoring is usually not performed in wind turbine generators and, thus, there is a risk of overheating the rotor windings of the generator. Especially so, because the power in the rotor windings varies during operation of the wind turbine, which makes the variations of the temperature in the rotor difficult to predict if it is not being monitored.
There are at least two major problems related to the commonly used control strategy associated with the above-mentioned monitoring of a series of parameters.
Firstly, the strategy normally consists of derating the power production of the wind turbine, whenever a certain safety limit of a monitored value is reached. The derating continues until another limit is reached, whereupon the wind turbine returns to normal operation. Such a control strategy will prevent the temperature from exceeding a certain maximum value, but at the same time, it causes the temperature to fluctuate within a certain range, if the wind turbine is operating with generator temperatures close to their maximum limits. Thus, overheating but certainly not temperature variations is avoided.
Secondly, the monitored parameters are typically controlled individually. This means that the safety limits set up for the values of a given parameter must be very conservative in order to make sure, that overheating does not occur, because the actual values of other parameters affecting the generator temperature are not taken into consideration. This leads to a non-optimized power production, since the production is derated if only one of the monitored parameters is at a critical level, although the actual generator temperature may be far from critical.
German Patent Application DE 33 42 583 discloses a method of controlling the power uptake of the rotor of a wind turbine by adjusting the rotor blades, as a part of which method it is proposed to monitor the temperature of the generator and to control the power uptake of the rotor as a function of this temperature in such a way that a critical generator temperature is not exceeded.
International Patent Application WO 02/086313 discloses a method for avoiding damp in a wind turbine generator by heating up the generator if the generator temperature is below the ambient temperature. Alternatively, the heating of the generator can be trigged by some kind of humidity sensor.
German Patent Application DE 41 41 837 discloses an apparatus and a method for controlling a generator so as to achieve a larger power output performance without overheating the generator. The method includes measuring the temperature and calculating whether a given maximum temperature has been reached or is close to being reached. If this is the case, the excitation current of the generator is reduced in order not to exceed the maximum temperature.
German Patent Application DE 101 06 944 discloses a method for controlling the temperature of an electric machine. The method prevents a critical temperature from being exceeded at temperature-critical components by the use of control measures involving temperature measurement and/or modeling and regulation to reduce excessive temperatures.
None of the above-mentioned documents mention closed-loop regulation of temperature or the aim of keeping the temperature substantially constant.
An objective of the present invention is to provide an apparatus and a method for providing a control system for the temperature of a wind turbine generator that prevents overheating and reduces significantly the temperature variations and, at the same time, keeps the power production of the wind turbine at its optimum under the given operational and environmental conditions.